Schottky diodes are important passive components in, for example CMOS ICs that perform radio frequency (RF) and mixed signal (MS) functions. CMOS Schottky diodes include two terminals, an anode and a cathode, that are formed on a surface of the CMOS integrated circuit substrate (e.g., monocrystalline silicon), and also include isolation structure positioned between the anode and cathode. The Schottky diode consists a Schottky barrier, which is a metallic region, in direct contact with a relatively lightly doped semiconducting region, a method providing an ohmic contact to that lightly doped semiconducting region, which will be called a backside contact, and the structures necessary to define and isolate the two different contact regions. According to the choice of substrate doping and the metallic material, the metallic region may be either a cathode or an anode. When the metallic material is in contact with a P-type region, it takes the role of cathode. On the other hand, if the metallic material is in contact with N-type silicon, it takes the role of an anode. In either case, the Schottky diode is completed with an ohmic contact to the underlying semiconductor region. When the Schottky diode is biased so that the anode is positive with respect to the cathode, and when a sufficient bias voltage exists between the Schottky barrier and the Ohmic contact, a relatively high current is produced that passes through the intervening substrate. When the anode is biased negatively with respect to the cathode, a much reduced, reverse current flows. Like all diodes, the Schottky diode is subject to breakdown if excessive reverse voltage is applied. The magnitudes of the forward and reverse currents are determined first by the choice of N-type or P-type semiconducting material, second by the choice of metallic material, third by the doping density of the semiconducting material, and finally by the details of the device geometry.
Due to the general trend toward RF and MS CMOS ICs that function at ever-lower operating voltages, there is a need for passive components, such as Schottky diodes, that exhibit a sufficiently low turn on voltage and low series resistance. These Schottky diode operating characteristics can be “tuned” to a desired level through the selection of either N-type or P-type doping for the semiconducting region, the selection of metallic material used to form the Schottky barrier and ohmic contact, the doping levels of the diode well, and the distance between the anode and cathode (i.e., the width of the isolation structure).
The present invention is particularly directed to reducing the series resistance of a Schottky diode in a way that minimizes interruption of a standardized CMOS process flow. In conventional Schottky diode fabrication, the isolation structure between the anode and cathode is formed using well-known shallow trench isolation (STI) or localized oxidation (LOCOS) techniques. Both LOCOS and STI structures are used in a CMOS process flow to, for example, to electrically isolate individual components from neighboring components. Since the more advanced processes use STI, the balance of this discussion will address STI. A problem with using an STI structure as a Schottky diode isolation structure is that STI structures are relatively wide (when compared with other lithographically formed IC structures) and extend into the substrate, thereby creating a relatively long current flow path between the Schottky diode's anode and cathode. Because series resistance is directly related to this current flow path, it is difficult to reduce the series resistance of Schottky diodes formed with STI-based isolation structures. This problem is particularly severe with small diodes that are appropriate to high frequency RF applications.
What is needed is a method for fabricating Schottky diodes for RF and MS CMOS ICs in which series resistance is minimized in order to facilitate low voltage operation. What is also needed is a method for producing such Schottky diodes that can be incorporated into a standard CMOS processing flow and in a manner that reduces the number of additional processing steps, thus maintaining the lowest production cost possible for such a diode and integrated circuits incorporating the diode.